This invention relates to metal vapor arc discharge lamps. More particularly, this invention is concerned with an end sealing structure for the arc tube of a metal vapor arc discharge lamp.
The sealing of high operating temperature sodium resistant arc tubes for use in metal vapor arc discharge lamps, such as, for example, high pressure sodium lamps, is a continually changing and improving art. The driving forces for such changes include lower cost, improved reliability, easier assembly and reduced shrinkage of product. Several different designs and modifications of these designs are presently in use. Three of these designs are illustrated in FIG. 1(a-c). These general designs can be adapted for use with an electrode assembly 9 employing either a tube or a wire electrical feed-through. Each modification has specific advantages and disadvantages.
The monolithic design (FIG. 1a) has been extensively used because of its minimal interior seal material exposure associated with only one seal region. It has the disadvantage though of requiring a more complex and costly arc tube, because the insert buttons required for making the two monolithic ends, must be precisely aligned and sintered in place.
The hat (FIG. 1b) and disk (FIG. 1c) seals take advantage of a less expensive straight tube construction. Unfortunately, while having cost advantages, they introduce other disadvantages. For example, these constructions require sealing of two regions, an inner and an outer annulus, which requires supplying seal material to the two regions.
The existing hat seal design (FIG. 1b) makes use of two different sealing rings 13, 14. Reference to FIG. 1b shows that the outer ring 14 is of a large diameter with a small cross-section; this is a fragile and easily broken piece. During assembly this frail construction is more prone to breakage with handling. More extensive handling is required to align the seal ring 14 on the hat 15 and then properly seat the hat into the arc tube 16. It has also been found that although the ring may appear to be intact, when it is heated near its melting point the ring can break from the added thermal stress. Breakage at this point can result in loss of part of the ring and thus insufficient sealing material is left to fill the outer annulus. This design also results in the two sealing rings melting at different times because of temperature variations during sealing. To achieve more uniform melting of seal material, sealing rings of different materials having different melting points have been used. This has the disadvantage of requiring preparation and handling of two materials with different chemical formulations, which adds cost to a manufacturing process.
Another problem frequently encountered with the existing hat seal design relates to proper seating of the hat during sealing. As can be observed in FIG. 1b, prior to melting of the outer seal material ring 14, the hat button is lifted off the arc tube by the thickness of the seal material ring 14. During sealing, the button must seat down into the arc tube as the seal material melts. It is possible for the hat button to tip slightly as the seal material melts and not achieve the proper seating, especially if the arc tube-to-button tolerances are too close.
The disk seal design, shown in FIG. 1c, has the advantage of needing only one sealing material ring 17. The disk seal design, however, has decreased sealing reliability and increased tolerance control between the disk 18 and the tube 19. The disk seal utilizes a first cross-wire 7 which prevents the disk from falling off the electrode assembly. A second cross-wire 8 keeps the disk from falling into the tube and also provides a means of capillary transport of sealing material to both the inner and outer seal regions. See, for example, U.S. Pat. No. 4,034,252 issued to McVey on July 5, 1977. One problem with this construction is that if the delicate cross-wire is accidentally bent before or during assembly, the capillary action will be decreased or stopped entirely thereby resulting in incomplete filling of the outer annulus. The flow of sealing material into the outer annulus is also dependent on the width of the annulus. If it is too large, sealing material will reach the inner edge of the annulus, but not be able to bridge the gap because the cross-wire provides no capillary action across the gap. Another problem with this design relates to centering alignment of the electrode within the arc tube.
Examples of configurations used for the second cross-wire are illustrated in FIGS. 2 (a-c). The simplest cross-wire design, shown in FIG. 2a, provides only two points of contact 21, 22 between the disk 23 and the single support wire 24 which leaves the disk prone to rock.
Further modifications of the cross-wire configuration, such as the configurations shown in FIGS. 2b-2c, provide more stability but result in more complex parts assembly. FIG. 2b illustrates a dual wire configuration which employs two cross-wires 27, 28 for supporting the disk 23. The two cross-wires 27, 28 pass on opposite sides of the electrical feedthrough aperture 26. FIG. 2c illustrates a "hair pin" configuration. The "hair pin" configuration employs a single wire 29 which extends across the disk to one side of the electrical feedthrough aperture 26. The wire 29 is then bent to form a rounded or looped portion; the wire then extends back across the disk 23, passing the opposite side of the electrical feedthrough aperture 26. Again, the disk seal design shown in FIG. 1c requires an inner cross-wire or bend configuration to prevent the disk from sliding down on the feed-through.
Many of the aforesaid problems associated with existing seal designs result during the sealing operation. Failure to achieve the proper seal results in the loss of an arc tube as well as one or two costly electrode assemblies.